define readthedocs environment in terms of requirements files

[emlys]

Description

I recently learned that you can reference requirements.txt files from a conda environment.yml. Using that feature, we can deduplicate the ReadTheDocs environment file and be sure it's up to date.

• Move sphinx requirements from requirements-dev.txt into a new file requirements-docs.txt
• Rename the RTD environment file from requirements-docs.yml to .readthedocs_environment.yml
• In .readthedocs_environment.yml, point to the requirements files rather than duplicating the list of requirements
• In the RTD build config file .readthedocs.yml, update some deprecated syntax, and make use of the new jobs feature to install invest with make install. This is nice because it's consistent with our other CI builds.

In a second PR, I'd like to make some updates to the requirements files.

Checklist

• Updated HISTORY.rst (if these changes are user-facing)
• Updated the user's guide (if needed)
• Tested the affected models' UIs (if relevant)

[emlys]

Lets also squash these commits before merging

[phargogh]

Superseded by a merge conflict resolution pr in #1059

Sorry everyone! I accidentally closed this PR when I shouldn't have. Too many tabs open.

[davemfish]

Nice!